Ischemia is a reduction of blood flow, e.g., in the brain, that can result in metabolic impairment and neurological dysfunction. Ischemia in the brain typically results in a stroke. One of the research goals in stroke imaging is to identify the imaging correlate to the ischemic penumbra, tissue that is oligemic, i.e., has a lower blood volume, but is still viable. It is this tissue that has been speculated to maximally benefit from therapeutic intervention. An accurate in vivo measure of cerebral blood flow (CBF) may help guide therapeutic strategies on an individual patient basis.
Perfusion weighted SEW) magnetic resonance (MR) imaging, in which one tracks the passage of a bolus of a contrast agent, has been demonstrated to be useful for detecting tissue at risk of infarction. For example, see “Magnetic resonance imaging of acute stroke” by Baird et al., J. Cereb. Blood Flow Metab., 18:583-609 (1998). In general, contrast agent-based CBF maps are estimated by deconvolving tissue concentration curves with an arterial input function (AIF). This technique has been shown to be both sensitive to delay and dispersion between the AIF and the tissue concentration curve. For example, refer to “Modeling cerebral blood flow and flow heterogeneity from magnetic resonance residue data,” by Østergaard et al. in J. Cereb. Blood Flow Metab., 19:690-699 (1999) and “Delay and dispersion effects in dynamic susceptibility contrast MRI: simulations using singular value decomposition,” by Calamante et al. in Magn. Reson. Med., 44:466-473 (2000). Because perfusion maps depend on the selection of the AIF, determination of a threshold of irreversible ischemia is complicated and makes identification of tissue at risk of infarction sensitive to user variability. Furthermore, such artifacts may negatively affect patient management. For example, a vascular territory whose feeding artery is occluded will suffer delayed flow as well as reduced flow. However, even if the territory has sufficient collateral flow, the tissue may have an estimated CBF that is artificially oligemic due to these technical limitations.